A known implement having at least two axes and which is operated by providing control about the axes is a loader/bucket arrangement of the type used on tractors, skid-steer vehicles, articulated vehicles, backhoes, and tracked vehicles. Such an arrangement typically includes two loader arms pivotally attached to the vehicle at one end of the arms, and a bucket pivotally attached to the distal end of the arms. The loader arms are typically pivoted relative to the vehicle by hydraulic cylinders appropriately attached thereto to raise and lower the bucket. The bucket is pivoted relative to the arms by hydraulic cylinders appropriately attached thereto.
The power to actuate the hydraulic cylinders which produce the pivoting motion of the loader arms and of the bucket about their respective pivot axes is provided by pressurized hydraulic fluid supplied to the hydraulic cylinders by an appropriate pump or pumps driven by the vehicle engine, with the amount of available flow depending on engine speed. The flow of hydraulic fluid is controlled by valves which may be operated manually, electrically, or electromechanically. The valves for controlling the flow may also be pilot-operated hydraulic valves.
For many uses of loaders, it is desirable to maintain the orientation of the bucket relative to the surface upon which the associated vehicle is operating, or relative to the frame of the vehicle, as the loader arms are being raised or lowered. To achieve this result in certain conventional systems, the operator must manually control the valve for the hydraulic cylinders of the loader arms (i.e., "Arm Valve") while simultaneously controlling the valve for the hydraulic cylinder of the bucket (i.e., "Bucket Valve"). This simultaneous manual control over the Arm and Bucket Valves requires that the operator maintain visual contact with the bucket, which on certain vehicles is difficult. In many situations, the vehicle and loader configuration do not permit the operator to properly determine the orientation of the bucket over the full range of motion of the arm and bucket. In addition, manual control over both the Arm and Bucket Valves to maintain the bucket orientation relative to the surface, or the frame, increases the workload on the operator, resulting in increased operator fatigue and decreased operator capacity to control other vehicle and loader functions such as driving the vehicle. Further, manual control over both the Arm and Bucket Valves is subject to errors associated with any manual control operation, resulting in decreased control accuracy. For example, errors which result from manual control of both the Arm and Bucket Valves can result in rolling the bucket too much as the arms are raised and lowered, resulting in spillage of the load.
In response to this need for a loader arrangement which can maintain the orientation of the bucket relative to the surface over which the arm is raised and lowered, or relative to the vehicle frame, loaders have been designed to include self-leveling linkages which maintain the orientation of the bucket relative to the surface or to the vehicle frame. Alternatively, some loaders have been designed to combine the operation of the Arm and Bucket Valves to provide improved bucket orientation control. One problem with many of the presently used arrangements for bucket orientation control is the complexity of such arrangements. This complexity increases cost and in most cases, reduces reliability. Another problem with certain existing systems is the utilization of operator controls which are not easily and efficiently manipulated by the operator to achieve desired loader operations. Another existing system includes hydraulic leveling valves inserted between the Arm and Bucket Valves and the cylinders. As the arm is commanded to raise and lower, these leveling valves automatically roll the bucket to maintain the bucket level. However, these leveling valves are expensive, and have a relatively poor performance since the bucket is often allowed to drift from its level orientation.
In view of the need for improved bucket control and the drawbacks of existing systems, it would be desirable to provide an improved electronic system usable by an operator to effectively control the orientation of the arms and bucket of a loader or other implement requiring coordinated control about at least two axes. Such an automatic attitude control system for controlling bucket orientation would reduce operator workload, decrease operator fatigue, and increase control accuracy. Such a system can also be used for controlling anti-rollback and return-to-position.
In electrohydraulic systems, the amount of fluid flow from the engine-driven hydraulic pump effects how much the hydraulic valves need to be opened or closed to obtain a desired angular velocity of the loader arms and bucket. At times, there is not enough flow from the engine to achieve the desired velocity. Although it is possible to increase the power of the engine and pump to increase the available flow, such increases are expensive. Further, the operator of such vehicles may, at times, set the engine throttle low to reduce fuel consumption and/or noise, which will also result in a decrease in the available flow. In situations where the desired amount of fluid flow of multiple hydraulic actuators exceeds the available amount of fluid flow, some or all of the hydraulic actuators may become starved, resulting in improper and unexpected controller operations.
Further, even in cases where there is sufficient available fluid flow, and even though the closed-loop control of existing systems can adapt to changing flow levels, there will be some conditions (e.g., high engine speed with full throttle) where the valves will not be required to be open as much as normally, and there will be other conditions (e.g., low engine speed with low throttle) where the valves will need to be open further than normal. In existing systems, the controller cannot determine which situation the flow is in using only the information from the position sensors for the arm and the bucket. Thus, prior art controllers require high gain to allow the controller to make large corrections to account for changes in the amount of flow. With such high gain systems, however, problems with stability arise which cause, for example, oscillation. Therefore, there is a need for an improved arm and bucket controller that measures the engine speed and determines the available flow based at least partly on engine speed, such that the controller can use a smaller gain, thereby increasing the stability of the system and providing more accurate control.
Prior bucket control systems use velocity-based control, where the controller attempts to control angular velocity of the loader arms and bucket based upon a velocity command depending upon the position of a command device. In such velocity-based controls, however, there may be either too much error (e.g., the bucket may fail to reach a level orientation after being moved, such that position accuracy is poor), or the bucket orientation is not stable (e.g., the bucket position may oscillate, even though the position accuracy may be better). Thus, in prior bucket control systems, it is difficult to achieve the desired system accuracy and stability requirements due to the trade-off which must be made between the control accuracy and control stability, depending upon whether the gain is higher or lower.
Thus, it would also be desirable to provide a flow-based control that increases stability (i.e., eliminates oscillation) while reducing error (i.e., increasing position control accuracy) under all operating conditions of the system. It would also be desirable to have a flow-based control capable of determining the available flow, and limiting the commanded flows to avoid exceeding the available flow.